Generation of protective immune responses against many potential pathogens requires the direct interaction between T cells and B cells. Central to these interactions is cognate recognition wherein T cells recognize antigenic peptides associated with MHC class II molecules displayed by B cells. During cognate interactions T cells provide important signals to B cells, "helping" them to proliferate and differentiate into antibody secreting cells. While the mechanism of delivery contact dependent helper signals is poorly understood, CD40L binding to CD40 on B cells has been advanced as a major mediator of these signals. However, studies using gene ablated mice indicate that other receptor-ligand pairs must be operative, particularly early in the collaborative response of T and B cells. We have recently shown that TCR binding to MHC class II molecules on antigen experienced B cells leads to transduction of signals via associated Ig-a/Ig-a heterodimers. These transducers were previously thought to function only as transducers of antigen receptor signals. MHC class II molecules associate with a unique cohort of Ig-alpha/Ig-beta, one that is not associated with antigen receptors. In this application we propose to define the role of MHC class II - Ig-alpha/beta association in generation of humoral immune responses by studying knockin mice that express mutant class II molecules incapable of Ig-alpha/beta association (aim 1). It is hypothesized that class II Ig-alpha/beta -mediated signals are important early in the immune response for clonal expansion and generation of primary B cell foci. In aim 2 we will determine why, despite their usage of the same transducers, biologic responses mediated by antigen receptors and MHC class II differ. We will pursue preliminary findings that these receptors utilize distinct co-receptors. Finally, studies in aim 3 will determine the molecular basis of MHC class II - Ig-alpha/beta interaction in primed cells, testing the hypothesis that association is mediated by a linker/adaptor molecule. By defining molecular signaling circuitry involved in T cell-dependent B cell activation, these studies may reveal sites of molecular defects causative in immunodeficiency or autoimmunity. In addition they may lead to novel strategies for improved vaccination and immunosuppression.